Nuclear magnetic resonance spectroscopy (NMR) has been an important analytical technique since it became available in 1946. When atomic nuclei are placed in a constant, homogenous magnetic field of high intensity and subjected at the same time to a certain selected frequency RF alternating field, a transfer of energy can take place between the radio frequency field and the atomic nuclei to produce what is pictured as a flipping of the nuclei, which nuclei will immediately relax, i.e. flip back where they can reabsorb again resulting in a flipping back and forth. This flipping is called a resonance. More precisely, when a system of nuclei are exposed to radiation of frequency f.sub.o, such that the energy hf.sub.o of a quantum of radiation, where h equals Plank's constant, is exactly equal to the energy difference between two adjacent nuclear energy levels, then energy transitions may occur in which the nuclei may be pictured as flipping back and forth from one allowed orientation to another. The apparatus for such NMR experiments is relatively simple in concept and comprises a large magnet to create a fixed field H.sub.o and electronic equipment to generate RF excitation energy (transmitter) which is coupled to an excitation coil which is positioned around and excites a sample and an electronic receiver which is also coupled to the coil in a part of the spectrometer known as the probe. In modern NMR spectrometer equipment the receiver and transmitter are ordinarily turned on and off very quickly so that the receiver is not receiving when the transmitter is on and vice versa.
It is important that the receiver coil be very closely coupled to the sample atoms which are normally dissolved in a solvent and placed in a sample test tube and that the receiver coil be filled as much as possible with sample material, i.e., have a large filing factor, because the intensity of the received signal in the receiver coil is related to the number of nuclei that couple to the coil and because the emitter signal from each nucleus is very small. A large proportion of the total NMR spectrometer improvement effort in recent years has been spent in improving the probe part of the NMR spectrometer instrument to improve the techniques for coupling to the sample.
Early in NMR development it was appreciated that a most important parameter in NMR was the homogeneity of the DC magnetic field, H.sub.o. In fact when the homogeneity was first improved from the earliest magnets, the ability of the NMR spectrometer to perform high resolution spectroscopy was discovered. High resolution spectra are those spectra where the resonance lines are narrower than the major resonance shifts caused by differences in the chemical environment of the observed nuclei, such as are caused by secondary magnetic fields of the molecules of a sample. Homogeneity is a quality of the DC magnetic field. The goal is to have perfect homogeneity, meaning that all atoms of the sample are influenced by a magnetic field having identical direction and magnitude.
It has been heretofore appreciated that elements of the probe can also influence the homogeneity of the DC field lines across the sample. Since the probe contains different materials and shapes, each material and its shape influences the magnetic field differently and can accordingly affect the homogeneity deleteriously.
A prior patent by Anderson et al., U.S. Pat. No. 3,091,732 sought to make the probe coils and adhesives therefore from materials in which the mixed magnetic susceptibility matched that of the air surrounding the sample tube. Other inhomogeneity correction probe patents include U.S. Pat. No. 4,517,516 to correct for holes in a coil form, and U.S. Pat. No. 4,077,002 was directed to correcting for edge effects when an insufficient amount of the sample liquid itself was available to completely fill the observation area and well beyond. U.S. Pat. No. 4,549,136 teaches use of plugs of materials to be added to the sample tube whose susceptibility is equal to the sample tube solvent.
It has been standard for several years to employ long small diameter cylindrical sample tubes to conserve sample. When only very small sample volume is available, or the sample is very expensive even this approach is problematical and requires dilution of sample, with concomitant reduction in signal. In such cases sample plugs have been used in order to avoid end effects when the sample does not fill the sample space and beyond a satisfactory amount.
It was also known in the prior art to introduce a nylon plug into a sample tube which provided a spherical volume sample space. Also known were spherical glass bulbs. Spherical volumes have poor filling factor and suffered from imperfections in the glass of the sphere as well as perturbations introduced by the sample in the glass stem attached to the glass bulb.
It is an object of the present invention to enable improved magnetic field homogeneity and improve filing factor in an NMR sample probe by providing an improved shape of a sample with a reduced volume of sample.
It is a feature of the current invention that the diameter of the sample tube outside of the observe region decreases in diameter pursuant to a shape defined as a solid of revolution.
It is a further feature that the region of symmetrical shape of said sample tube on both sides of the observe region extend substantially further from said observe region for the same volume of sample liquid as in the prior art sample tubes.
It is a still further feature that appropriately configured plug inserts can be inserted into standard sample tubes to achieve the desired shape of the sample.